Thermal transfer printing dyesheet

ABSTRACT

A dyesheet for thermal transfer printing comprising a thermoplastic substrate film supporting a dyecoat containing a thermal transfer dye on one surface and a heat resistant backcoat on the other, wherein the backcoat comprises the following components: 
     a) a crosslinked polymeric binder having a thickness t and containing therein a combination of 
     b) lubricating particles and 
     c) load-bearing particles having an average diameter greater than t, 
     and the haze value is less than 12%.

The invention relates to dyesheets for forming printed images onreceiver sheets by thermal transfer of dyes, using such heating means asthermal heads controlled by electronic image signals; and in particularto heat resistant backcoats therefor.

Thermal transfer printing is a process for generating printed images bytransferring thermally transferable dyes from a dyesheet to a receiver.The dyesheet comprises a base sheet coated on one side with a dyecoatcontaining one or more thermally transferable dyes, and printing iseffected while the dyecoat is held against the surface of the receiver,by heating selected areas of the dyesheet so as to transfer the dyesfrom those selected areas to corresponding areas of the receiver. Thisgenerates an image according to the areas selected. By repeating thetransfer process with dyesheets of the three primary colours, fullcolour images can be obtained. Further panels, e.g. black, may also beprovided.

Thermal transfer printing using a thermal head with a plurality of tinyheaters to heat the selected areas, has been gaining widespreadattention in recent years, mainly because of its ease of operation inwhich the areas to be heated can be selected by electronic control ofthe heaters (e.g. according to a video or computer-generated signal),and because of the clear, high resolution images which can be obtainedin this manner.

The base sheet of a thermal transfer dyesheet is usually a thinthermoplastic film, generally orientated polyester film on account ofits superior surface smoothness and good handling characteristics. Thethermoplastic materials used in such films, however, may lead to anumber of problems. For example, for high resolution printing at highspeed, it is necessary to provide the thermal stimulus from the heatersin pulses of very short duration to enable all the rows to be printedsequentially within an acceptably short time, but this in turn requireshigher temperatures in the printer head in order to provide sufficientthermal energy to transfer sufficient dye in the time allowed. Suchtemperatures may be well in excess of the melting or softeningtemperatures of the thermoplastic base sheet, typically rising to300°-400° C. during pulses of a few milliseconds. One adverse effect ofsuch high temperatures can be localised adhesion between the dyesheetand the printer head, with a result that the dyesheet is unable to bemoved smoothly through the printer, and in severe cases the base sheetcan lose its integrity, with tearing of the dyesheet resulting.

These problems are usually addressed by providing the dyesheet with oneor more protective backcoats of various heat-resistant, highlycrosslinked, polymers. By "backcoats" in this context we mean coatingsapplied either directly or indirectly on the base sheet surface remotefrom that to which the dyecoat is applied. Thus it is to the backcoatside to which heat is applied by the thermal head during printing. Inaddition to providing a heat resistant layer to combat sticking,backcoats may also be formulated to improve slip and handlingproperties.

Poor slip and handling properties can lead to printing defects such asribbing, and smiles. "Ribbing" is the appearance of lines transverse tothe movement through the printer, which normally extend the full widthof the print. They are formed by longitudinal variation in the opticaldensity of the print, and occur when there are variations in the amountby which the dyesheet and receiver feed through the printer after eachrow of pixels has been printed. "Smiles" are short, usually curved,transverse lines caused by creasing of the dyesheet as it passes thoughthe printer.

These problems have previously been attacked by adding heat resistantparticles to stand proud of the binder surface, together with one ormore lubricants and/or surfactants, but inappropriate slip/handlingadditives can also lead to the printed image having low colour density,streaks and/or indentations in the direction of travel of the receiversheet through the printer, often referred to as "scratching", from itsappearance.

Compositions of backcoats comprising crosslinked binders containing acombination of load bearing particles with lubricants and/orsurfactants, are found for example in EP-A-314,348, which describes theuse of talc particles with long alkyl chain lubricants such as zinc andlithium stearates and a surfactant, and EP-A-458,522 which similarlyuses talc particles and surfactant, but with salts of long chain alkylesters of phosphoric acid such as zinc stearyl phosphate. The specificembodiments exemplified in these two publications comprised binderscontaining variously about 9-17% by weight of the additives.EP-A-329,117 gives long lists of widely differing types of compoundsfrom which the particles and the lubricant/surfactants respectively maybe selected, and the Examples describe several very differentcompositions, including one using particles of polymethyl silsesquioxane(Tospearl 120) with a silicone surfactant (NUC silicone L7602) at acombined level of about 27% by weight of the binder resin. The use oflarge spherical particles such as Tospearl 120, is also described inEP-A-411,642, but in combination with mineral particles less than 10%the size of the large particles.

From the many end diverse compositions that have previously beenproposed, the above examples of prior art have been selected withhindsight of the present invention, there being also a wealth of otherproposed compositions that use additives different from those employedhere. Also, in an earlier copending application, EP-A-547,893, we havedescribed dyesheet backcoats of crosslinked acrylic binders containing acombination of polymethyl silsesquioxane particles and a particulatesalt of a higher fatty acid or higher fatty acid phosphate, the specificembodiments containing the additives in amounts of about 11% by weightof the binder, or higher. That new combination of selected binder, loadbearing particles and lubricant particles provided an unexpectedly goodbalance of slip and handling properties without the scratching and longterm storage stability problems associated with some other previouslyproposed combinations.

However, we have noticed that at least some of the above dyesheets arenot totally compatible with some, but certainly not all, commerciallyavailable printers, which then fail to operate consistently. We have nowtraced this to haze in the backcoat scattering light from sensors in theprinters and causing them not consistently to detect location marksand/or dye sequence changes in the dyecoat. (As a measure of haze inthis context, we use a Gardner XL 211 Hazeguard System, and the valuesquoted for haze herein are the values obtained or obtainable by thissystem.)

According to the present invention a dyesheet for thermal transferprinting comprises a thermoplastic substrate film supporting a dyecoatcontaining a thermal transfer dye on one surface and a heat resistantbackcoat on the other, wherein the backcoat comprises the followingcomponents:

a) a crosslinked polymeric binder having a thickness t and containingtherein a combination of

b) lubricating particles and

c) load-bearing particles having an average diameter greater than t, andthe haze value is less than 12%.

A wide variety of highly crosslinked polymer compositions havepreviously been proposed for backcoat binders (component a), but forachieving low haze in the backcoat when using the particulate solids(components b & c) described in detail hereinafter, we prefer to usecrosslinked acrylic compositions based on one or more polyfunctionalorganic resins having from 2 to 8 pendent or terminal acrylic ormethacrylic groups per molecule available for crosslinking. These may beapplied as monomer or oligomer solutions to the base film surface, andthereafter crosslinked so as to form a strong heat-resistant layer.

Examples of polyfunctional acrylic compounds include 1,6-hexandioldi(meth)acrylate (the designation "(meth)" being used herein to indicatethat the methyl group is optional), ethylene glycol di(meth)acrylate,trimethylol propane tri(meth)acrylate pentaerythritol tri(meth)acrylate,pentaerythritol tetra(meth)acrylate, and dipentaaerythritolhexa(meth)acrylate, and esters of (meth)acrylic acid with polyesterpolyols and polyether polyols which are obtainable from a polybasic acidand a polyfunctional alcohol, urethane (meth)acrylates obtained throughe reaction of a polyisocyanate and an acrylate having a hydroxy group,and epoxy acrylates obtained through a reaction of an epoxy compoundwith acrylic acid, an acrylate having a hydroxy group or an acrylatehaving a carboxyl group.

These polyfunctional resins can be used in combination with linearorganic polymers, which do not copolymerise with them duringcrosslinking but which have an effect on the physical properties of thecoating. Examples include polymethylmethacrylate and polyvinylchloride.

Instead or in addition to the linear organic polymers, thepolyfunctional acrylic resins can be copolymerised with at least oneorganic compound having a single acrylic or methacrylic group permolecule.

Examples of suitable monofunctional compounds include such aliphatic(meth)acrylates as 2-ethylhexyl (meth)acrylate and lauryl(meth)acrylate, such alicyclic (meth)acrylates as cyclohexyl(meth)acrylate, isobornyl (meth)acrylate, dicyclopentanyl(meth)acrylate, and dicyclopentadienyl (meth)acrylate, suchalkoxyalkylene glycol (meth)acrylates as methoxydiethylene glycolacrylate, and ethoxydiethylene glycol acrylate, such aromatic(meth)acrylates as phenyl acrylate, and benzyl acrylate, and such(meth)acrylates of aliphatic alcohols as 2-hydroxyethyl (meth)acrylate,and 2-hydroxyethyl di(meth)acrylate. Of these, compounds having at leastone alicyclic group per molecule are particularly favoured because oftheir low shrinkage characteristics, their resistance to migration ofthe dye from dyecoat to backcoat during storage end because they givecoatings with good heat resistance.

Backcoats are preferably as thin as possible conducive with theirproviding sufficient thermal protection and handling properties, inorder to minimise dissipation of the heat from the thermal head. Thiscan be severe at 2.5 μm for high resolution prints, and we prefer thebinder thickness to be not more than 2 μm. Most presently known bindercompositions require minimum binder thicknesses of 0.4 μm for adequateprotection, but the present haze and particle size criteria should stillbe equally valid for thinner compositions were these to become feasible.

Preferred lubricating particles (component b) are carboxylic orphosphoric acids, acid amides, esters and their multivalent metal salts,with at least one C₁₂₋₃₀ alkyl chain. Examples include particles ofstearic acid and its multivalent metal salts, especially calciumstearate, magnesium stearate, zinc stearate and aluminium stearate,stearamide, behenic acid and its multivalent metal salts, especiallycalcium behenate, magnesium behenate, zinc behenate and aluminiumbehenate. Other examples include multivalent metal salts of phosphateesters expressed by the following general formula (A) and (B): ##STR1##in which R is an alkyl group of C₁₂₋₃₀ or an alkylphenyl group, m is anintegral number of 2 or 3, and M a metal atom. Preferred examples ofsuch salts include zinc stearyl phosphate, zinc lauryl phosphate, zincmyristyl phosphate, calcium stearyl phosphate, magnesium stearylphosphate, barium stearyl phosphate, aluminium stearyl phosphate,aluminium lauryl phosphate and aluminium tridecyl phosphate.

We find that components b and c both contribute to haze values, and thatthe larger the size of particles used, the greater tends to be theresultant haze. The smallest lubricant particles (component b) that wehave been able to obtain, have produced lubrication not detectably worsethan that produced by the larger particles (indeed they have generallyappeared to provide enhanced lubrication), but the haze values do tendto be noticeably lower with the smaller particles, enabling largeramounts to be used for better printing properties but still with lowhaze. It appears that the smaller the size available, the better will bethe result. We have used lubricants down in size to 0.2 μm withbenefits, and sizes down at least to 0.1 μm seem preferable, with acommon particle size of 2.5 μm providing a suitable upper limit abovewhich haze values tend to intrude.

For the load-bearing particles (component c), we prefer to use sphericalparticles, examples of which include silsesquioxane compounds. Thesilsesquioxane structure means one wherein each of three bondings of asilicon atom are directly bound to oxygen atoms to form athree-dimensional crosslinked structure, wherein the single remainingbonding is substituted with a C₁₋₁₇ alkyl group which can be branched orunbranched, alkylsilyl group, silylalkyl group, aryl-substituted alkylgroup, amino group, epoxy group, or vinyl group. Polymethylsilsesquioxane compounds that can readily be obtained include Tospearl105, Tospearl 108, Tospearl 120, Tospearl 130, Tospearl 145 and Tospearl240 (Toshiba Silicone products), and KHP-590 (Shinetsu Chemicalproduct).

Other materials which can be used as load-bearing particles (componentb) include silicone gel elastomers, commercially available examples ofwhich include Torefil E 730S and Torefil E 500 (Toray Dow Corningproducts), and low surface energy particles such as polymers andcopolymers of fluorinated alkenes, especially polytetrafluoroethylene(PTFE).

The size of the load bearing particles (component c) is governed by theneed for these to stand proud of the backcoat resins, and averageparticle diameters of 1.2 t-2 t are preferred. However, particles aslarge as 4 t can be used without exceeding the above haze values, whenused with particularly small lubricant particles (component b).

For minimum haze it is also desirable to use the least amount of the twosets of particles effective to give adequate slip and handlingproperties. We have now found that the above described lubricants andload bearing particles, when used in combination, enable lower particlelevels to be used, while still retaining good slip end handlingproperties. Proportions of the two species of particles together may beas low as 1.5% by weight of the binder when using the above preferredspecies of particles (b & c) in combination, without too muchdeterioration of the printing performance. However, both lubricatingparticles (b) and load bearing particles have important roles to play,and we prefer that each of the species of particles (b & c) are presentas at least 0.5% by weight of the binder.

If the haze level is to be kept within the specified values, it isdesirable to use not more the particles than 6% by weight of the binder,unless the lubricant particles predominate and have an average diameterless than about 1 μm, when an upper limit about 8% by weight of thebinder may still provide a haze value within the limits specifiedherein.

The amounts of each of the two components need not be the same. Ourpreferred backcoat contains the lubricating particles (b) and loadbearing particles (c) in the weight ratio (b:c) of 1:1 to 10:1. Wherethe ratio is 6:1 or greater, however, it is preferred that the lubricantparticle size be about 1 μm or less.

A particularly preferred dyesheet for achieving such low haze values isone wherein the backcoat comprises a crosslinked polymeric binder (a)having a thickness t and containing therein a combination of lubricatingparticles (b) selected from at least one carboxylic or phosphoric acid,acid amide, ester and multivalent metal salts thereof, each having atleast one C₁₂₋₃₀ alkyl chain and an average particle diameter of 0.1-2.5μm; and load-bearing particles (c) which are at least one of sphericaland elastomeric, with an average particle diameter of 1.2 t-2 t; andwherein the proportions by weight of components a, b and c are given bythe formula: b+c/a=0.015 to 0.08.

According to a further aspect of the invention, there is provided amethod of thermal transfer printing by transferring thermallytransferable dyes from a dyesheet to a receiver using a printer havingat least one sensor susceptible to excess haze in the dyesheet, whereinthe dyesheet has a backcoat with a haze value of less than 12%, andcomprises a crosslinked polymeric binder (a) having a thickness t andcontaining therein a combination of lubricating particles (b) selectedfrom at least one carboxylic or phosphoric acid, acid amide, ester andmultivalent metal salts thereof, each having at least one C₁₂₋₃₀ alkylchain and an average particle diameter of 0.1-2.5 μm; and load-bearingparticles (c) which are at least one of spherical and elastomeric, withan average particle diameter of 1.2 t-2 t; and wherein the proportionsby weight of components a, b and c are given by the formula: b+c/a=0.015to 0.08.

EXAMPLES

The invention is now illustrated by reference to dyesheets prepared fromspecific compositions in which the proportions of the lubricatingparticles and the load bearing particles were varied and the resultscompared.

Examples 1-6 and Comparative Examples 1' and 2'

In each of these, a backcoat of about 1 μm dry film thickness wasobtained by uniformly coating the following backcoat compositions ontoone surface of a 6 μm polyester film (Lumirror, Toray product) using aNo 3 wire bar, drying for 10 seconds with a dryer, and then curing byirradiation from 15 cm distance using a 80 W/cm ultraviolet beamirradiation apparatus (UVC-254, Ushio product). The values for b and cin the composition were varied from one Example to the next, and theamounts are given in Table 1 below. All quantities are quoted as partsby weight.

    ______________________________________                                        Backcoat composition                                                          ______________________________________                                        Ebecryl 220      60          parts                                            isbornyl acrylate                                                                              26          parts                                            Diakon LG 156    14          parts                                            zinc stearate (2 μm)                                                                        b           parts                                            Tospearl 120     c           parts                                            Atmer 129        1           part                                             Quantacure ITX   1.7         parts                                            Quantacure EPD   1.7         parts                                            Irgacure 907     3.4         parts                                            methyl isobutyl ketone                                                                         150         parts                                            ______________________________________                                    

where: Ebecryl 220 is a 6 functional radical polymerisable urethaneacrylate from Daicel UCB), isbornyl acrylate is a monofunctional radicalpolymerisable compound, Diakon LG 156 is a polymethyl methacrylateproduct from ICI, Atmer 129 is an antistatic agent from ICI, Tospearl isa polymethyl silsesquioxane silicone resin powder having a mean particlesize of 2.0 μm from Toshiba, Quantacure ITX is a photoinitiator fromInternational Biosynthetics, Quantacure EPD is a photosensitizer fromInternational Biosynthetics, and Irgacure 907 is a photoinitiator fromCiba-Geigy

On the other side of the substrate was first applied a barrier layercomposition of the below-listed components, dried, cured and covered inits turn with a dyecoat composition comprising the components listedbelow, and dried to form a dyecoat about 1 μm thick.

    ______________________________________                                        Dye-barrier composition                                                       ______________________________________                                        Ebecryl 220       70         parts                                            Diakon LG 156     10         parts                                            Synocure 861X     20         parts                                            Quantacure ITX    1.7        parts                                            Quantacure EPD    1.7        parts                                            methyl isobutyl ketone                                                                          150        parts                                            ______________________________________                                    

Synocure 861X is an acrylated polyester polyol having zero radicalfunctionality.

    ______________________________________                                        Thermal transfer printing dyecoat composition                                 ______________________________________                                        Thermal transfer dye mixture                                                                     5.3        parts                                           PVB (BX1)          4.7        parts                                           ethyl cellulose (T10)                                                                            1.2        parts                                           tetrahydrofuran    90         parts                                           ______________________________________                                    

A receiver sheet was prepared based on a substrate of polyester film(Melinex 990, ICI product) of 100 μm thickness. A dye-receiving layercomposition was prepared using the below-listed components, which werethe coated onto one face of the substrate using a wire bar No 6, to givea dye-receiving layer of about 4 μm dry film thickness.

    ______________________________________                                        Dye-receiving layer                                                           ______________________________________                                        Vylon 200         100        parts                                            Tegomer HSi 2210  0.7        "                                                Cymel 303         1.4        "                                                Tinuvin 900       1.0        "                                                p-toluene sulphonic acid                                                                        0.4        "                                                toluene/MEK (60/40)                                                                             1000                                                        ______________________________________                                    

Tegomer HSi 2210 is a bis-hydroxyalkyl polydimethylsiloxane sold byGoldshmidt, cross-linkable by the Cymel 303 under acid conditions toprovide a release system effective during printing. Cymel 303 is ahexamethoxymethylmelamine from American Cyanamid. Nacure 2538 is anamine-blocked p-toluene sulphonic acid catalyst, and Tinuvin 900 is a UVstabiliser.

Samples of each of the dyesheets thus prepared were placed against areceiver sheet with dyecoat and dye-receiving layer in contact, andpassed through a number of printers in turn, such that each dyesheet wasevaluated for use in each of the printers. The results are summarised inTable 1.

                  TABLE 1                                                         ______________________________________                                               Formulation                                                                   % w/w               Printer Performance                                       Component   Haze    Ribbing                                            Example                                                                              b        c      %     i   ii    iii Smiles                             ______________________________________                                        1      1.5      1.0    7.8   4   3     4   none                               2      3.0      0.5    9.5   4   4     2   none                               3      3.0      1.0    9.7   2   2     2   none                               4      3.0      1.5    11.5  2   1     1   none                               5      4.0      0.5    10.3  3   4     2   none                               6      5.0      0.5    10.6  3   4     2   none                               1'     6.0      0      9.2   4   5     2   none                               2'     5.0      5.0    >30.0 1   1     1   none                               ______________________________________                                    

In Table 1, the printer performance is assessed by evaluating theribbing under three different conditions, thus:

i is a width step down, where a full width transverse band of highdensity is abruptly changed to two spaced narrow bands, repetitionsgiving a lattice print. Faults show as an unprinted line immediatelyafter each width reduction,

ii is a big area of maximum density. Faults show as transverse ribs, andpossibly also smiles,

iii is a power step down, where after printing a block at full power, anabrupt change to a lower power, half or less, is made. Faults show as aseries of transverse ribs, becoming progressively faintar in most cases,and

smiles are transverse arcuate areas of low optical density, and theseare looked for in areas of maximum density (is ii conditions).

Under "Printer Performance" the lower the number, the better was theperformance with respect to ribbing defects, with 1 signifying excellentperformance, 2 good, 3 acceptable, 4 fair and 5 poor performance.Example 1' is a comparative Example in which the load-bearing particlesare absent, and although the haze values were low, the printingperformance suffered, this showing most where large blocks of solid highdensity colour were required.

Printer compatibility

Different printers may react differently to hazy dyesheets. Some operatewithout problems, but others may miss some colour repeats. Of thelatter, some may stop after failing to detect two repeats, whereasothers just fail to print at all. The samples were tested on a number ofdifferent commercial printers, some of which we knew to be particularlyhaze sensitive, and others with which we had previously had no problems.

No such problems were experienced with any of the dyesheets of Examples1-6 and 1'. Example 2' is a further comparative Example using the samelubricant and load bearing particles, but in sufficient quantity to givea haze value greater than the 12% specified above. Compatibilityproblems as described above were experienced when using this dyesheet insome, but not all, of the printers tested.

Example 7

In this Example an ultrafine particulate lubricant was used.

    ______________________________________                                        Backcoat composition                                                          ______________________________________                                        Binder resins        95       parts                                           zinc stearate (ultrafine lubricant)                                                                3        parts                                           (average particle size 0.2-0.4 μm)                                         KMP-590 (load bearing particles)                                                                   2        parts                                           (average particle size 2.0 μm)                                             ______________________________________                                    

KMP-590 is a silicone gel sold by Shinetsu Chemicals. The binder resinswere essentially as described in the previous Examples, and weresimilarly crosslinked in situ by free radical polymerisation of theacrylic groups, to give a dry backcoat of about 1 μm thickness.

The haze value was again less than 12%, and no compatibility problemswere experienced with any of the printers. Excellent printingperformances (value 1 in Table 1 above) were obtained in each of theribbing tests.

I claim:
 1. A dyesheet for thermal transfer printing comprising athermoplastic substrate film supporting a dyecoat containing a thermaltransfer dye on one surface and a heat resistant backcoat on the other,wherein the backcoat comprises the following components:a) a crosslinkedpolymeric binder having a thickness t and containing therein acombination of b) lubricating particles and c) load-bearing particleshaving an average diameter greater than t,and a haze value of less than12%.
 2. A dyesheet as claimed in claim 1 wherein the crosslinkedpolymeric binder comprises a crosslinked acrylic composition based onone or more polyfunctional organic resins having from 2 to 8 pendent orterminal acrylic or methacrylic groups per molecule available forcrosslinking.
 3. A dyesheet as claimed in claim 2 wherein the acryliccomposition comprises at least one organic compound having a singleacrylic or methacrylic group per molecule, which is copolymerised withthe polyfunctional acrylic resins in forming the backcoat binder.
 4. Adyesheet as claimed in claim 1 wherein the the binder thickness is lessthan or equal to 2 μm.
 5. A dyesheet as claimed in claim 1 wherein thelubricating particles are carboxylic or phosphoric acids, acid amides,esters and their multivalent metal salts, with at least one C₁₂₋₃₀ alkylchain.
 6. A dyesheet as claimed in claim 5 wherein the lubricatingparticles are multivalent metal salts of phosphate esters expressed bythe following general formulae (A) and (B): ##STR2## in which R is analkyl group of C₁₂₋₃₀ or an alkylphenyl group, m is an integral numberof 2 or 3, and M a metal atom.
 7. A dyesheet as claimed in claim 1wherein the lubricating particles have an average particle diameter of0.1 to 2.5 μm.
 8. A dyesheet as claimed in claim 7 wherein thelubricating particles have an average particle diameter less than 1 μm.9. A dyesheet as claimed in claim 8 wherein the lubricating particlesand the load-bearing particles together are present as 1.5-8% by weightof the binder.
 10. A dyesheet as claimed in claim 1 wherein theload-bearing particles comprise spherical particles of silsesquioxanecompounds.
 11. A dyesheet as claimed in claim 1 wherein the load-hearingparticles comprise silicone gel elastomers.
 12. A dyesheet as claimed inclaim 1 wherein the load bearing particles have an average particlediameter of 1.2 t-2 t.
 13. A dyesheet as claimed in claim 1 wherein thelubricating particles and the load-bearing particles together arepresent as 1.5-6% by weight of the binder.
 14. A dyesheet as claimed inclaim 1 wherein the backcoat contains the lubricating particles (b) andload bearing particles (c) in the weight ratio (b:c) of 1:1 to 10:1. 15.A dyesheet for thermal transfer printing comprising a thermoplasticsubstrate film supporting on one surface a dyecoat containing a thermaltransfer dye and on the other surface a heat resistant backcoat, whereinthe backcoat has a haze value of less than 12% and comprises acrosslinked polymeric binder (a) having a thickness t and containingtherein a combination of lubricating particles (b) selected from atleast one carboxylic or phosphoric acid, acid amide, ester andmultivalent metal salts thereof, each having at least one C₁₂₋₃₀ alkylchain and an average particle diameter of 0.1-2.5 μm; and load-bearingparticles (c) which are at least one of spherical and elastomeric, withan average particle diameter of 1.2 t-2 t; and wherein the proportionsby weight of components a, b and c are given by the formula:

    b+c/a=0.015 to 0.08.


16. A method of thermal transfer printing by transferring thermallytransferable dyes from a dyesheet to a receiver using a printer havingat least one sensor susceptible to excess haze in the dyesheet, whereinthe dyesheet has a backcoat with a haze value of less than 12%, andcomprises a crosslinked polymeric binder (a) having a thickness t andcontaining therein a combination of lubricating particles (b) selectedfrom at least one carboxylic or phosphoric acid, acid amide, ester andmultivalent metal salts thereof, each having at least one C₁₂₋₃₀ alkylchain and an average particle diameter of 0.1-2.5 μm; and load-bearingparticles (c) which are at least one of spherical and elastomeric, withan average particle diameter of 1.2 t-2 t; and wherein the proportionsby weight of components a, b and c are given by the formula:

    b+c/a=0.015 to 0.08.